1. Field of the Invention
The present invention relates to lasers and, in particular, to tunable external cavity diode lasers.
2. Discussion of Related Art
An optical telecommunication system transmits information from one place to another by way of an optical carrier whose frequency typically is in the visible or near-infrared region of the electromagnetic spectrum. A carrier with such a high frequency is sometimes referred to as an optical signal, an optical carrier, light beam, or a lightwave signal. The optical telecommunication system includes several optical fibers and each optical fiber includes multiple channels. A channel is a specified frequency band of an electromagnetic signal, and is sometimes referred to as a wavelength. The purpose for using multiple channels in the same optical fiber (called dense wavelength division multiplexing (DWDM)) is to take advantage of the unprecedented capacity (i.e., bandwidth) offered by optical fibers. Essentially, each channel has its own wavelength, and all wavelengths are separated enough to prevent overlap. The International Telecommunications Union (ITU) currently determines the channel separations.
One link of an optical telecommunication system typically has a transmitter, the optical fiber, and a receiver. The transmitter has a laser, which converts an electrical signal into the optical signal and launches it into the optical fiber. The optical fiber transports the optical signal to the receiver. The receiver converts the optical signal back into an electrical signal.
External cavity diode lasers (ECDL) are attracting increasing attention with optical telecommunication system builders as potential widely tunable light sources. A typical ECDL includes a diode laser gain medium (i.e. a gain “chip” or semiconductor device) with an antireflection-coated facet on one end and a reflective or partially reflective facet on the other end, an end mirror, and a wavelength selection element (optical filter). The end mirror and reflective facet form an external laser cavity. The parameters of the gain medium are usually chosen to maximize the output power of the ECDL. The output power of the ECDL is maximized by, among other things, increasing the gain of the gain medium.
Laser resonators such as a gain medium have two distinct types of modes: transverse and longitudinal. Transverse modes manifest themselves in the cross-sectional intensity profile of the laser beam within the waveguide. Longitudinal modes correspond to different resonances along the length of the laser cavity which occur at different wavelengths within the gain bandwidth of the laser. Mode hopping occurs when relative intensities at different lines corresponding to different longitudinal modes shift under certain circumstances. In order to provide a reliable communications link utilizing an optical transmitter, it is desired to prevent mode hopping in lasers used in such optical communications applications.
A factor in whether mode hopping will tend to occur in a laser is the degree of stability of the laser. There are many forms of stability, including wavelength stability, pulse-to-pulse energy stability, repetition rate stability, thermal stability, bandwidth stability, among others, and these may be attempted to be controlled in various ways. For example, energy stability and repetition rate stability often depend on the stability of the electrical or optical energy input to the gain medium. The degree of wavelength or bandwidth stability may depend on quality of resonator materials and other factors. The degree of thermal stability may influence the wavelength or bandwidth stability, and may typically depend on the heat capacity of the gain medium and whether cooling and/or heating elements are provided along with a thermal sensor, i.e., a temperature controller, heat exchanger or other such thermal monitor and heat transport device, and what degree of sensitivity of thermal control these devices exhibit. Various developments have been made for stabilizing various parameters of laser systems including operating temperature, and preventing occurrences of mode hopping.
One characteristic of increasing gain medium gain to maximize ECDL output power is that the ECDL may become unstable in single mode and begin multimode lasing. This means that the ECDL begins lasing at different frequencies simultaneously or hops between several frequencies. Multimode lasing is acceptable in some applications, such as when the gain medium is used as a pump laser. However, when the gain medium is used in an application such as a tunable ECDL in an optical communication system such instability may be unacceptable. Customer specifications and telecommunications standards require that the diode laser must operate within a very tight range of frequencies and cannot hop from frequency to frequency.